A total of 2161 normal lactation records of Friesian cows kept at Sakha Farm of the Animal Production Research Institute, Ministry of Agriculture, Egypt during the period from 1978 to 2000 were used for measuring the level of inbreeding coefficients in this herd. The traits of concern were milk yield 305 days (305dMY), lactation period (LP), dry period (DP), days open (DO), lifetime milk yield (LTMY), total lactation period (TLP) and number of lactations completed (NLC). Inbreeding coefficients had no significant effect on 305 d MY, LP, DP and DO, but had highly significant effects on LTMY, TLP and NLC. A percent of 73.56 of cows were considered non inbred. Percent of Inbreeding depression per 1% increase of inbreeding was calculated to be -0.52 kg for 305dMY, -0.42 days for LP, -2.6 days for DP, +0.48 days for DO, -1.39 kg for LTMY, -1.65 days for TLP and +6.19 % lactation for NLC, when the level of inbreeding was from 0 to 6.25%. The corresponding percentages were -0.61 kg, -0.47 days, -1.37 days, +2.02 days, -2.8 kg, -2.42 days and +4.13 % lactation, respectively for the above traits, when the level of inbreeding was from 0 to 12.50%. The effects of inbreeding were cumulative and relatively larger on lifetime production traits than milk yield traits. This indicates the necessity for active management of the rate of inbreeding in dairy traits to monitor and control it in a breeding programme. The present study concluded that the losses due to inbreeding can easily be minimized through careful attention to identification of the mating and computerized mate selection programmes.